In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels and the like, the dried tissue web leaving the tissue machine is wound up into a large roll called a “parent roll”. These parent rolls are typically about 90-140 inches in diameter and 100-110 inches wide. Parent rolls are transported to a converting facility within the tissue mill where they are unwound and converted into the desired final product form. Among the various converting operations, fixed-load calendering of the unwound sheet to soften the sheet and control caliper (bulk) is commonly used.
In order to properly wind the dried tissue web into a parent roll, in some high-bulk processes, the tissue web is commonly over-dried to a moisture level of about 0.5-1.5 weight percent. If higher moisture levels are obtained, product quality issues can arise because of a loss of caliper in the parent roll and roll slippage during winding. Roll slippage often creates a roll that cannot be properly unwound in downstream operations.
On the other hand, producing a parent roll with a constant moisture level throughout the parent roll of about 0.5-1.5 weight percent requires more drying energy than is necessary for the final product because the dried sheet ultimately equilibrates to a moisture content of about 5 weight percent by the time it reaches the consumer. In addition, when the parent roll is formed, the wound-in pressure within the roll is inherently greater in the center of the roll than at the outer segments of the roll. This results in a reduced sheet caliper near the center of the roll as compared to the sheet caliper near the outer segments of the roll. This sheet caliper variation creates problems in the downstream converting/calendering operation because it results in sheet caliper variations in the final product.
Therefore, there is a need for a method for improving the caliper uniformity of calendered tissue sheets and for improving the energy efficiency of the tissue manufacturing process. This is particularly important for high-bulk, through-air-dried processes, where the web is often calendered to improve softness.